1. Field of the Invention
The present invention relates to a flexible shaft and method for manufacturing same. More specifically, it relates to a flexible shaft that has particular application as a coupling for transmitting power, for example, in bone drills, medullary reamers, flexible bone plug introducers, and other types of flex drivers.
2. Prior Art
Various types of flexible shafts are known for a variety of applications. In the medical field of orthopedics, medullary reamers are used to enlarge the medullary canals of bone in preparation for the insertion of fixation devices, performing an intramedullary osteotomy, stimulating bone growth, the insertion of a plug to preclude bone cement from migrating while it is in its viscous state, and for other reasons. The medullary canals of bone are seldom straight. More typically, the canal will have some degree of curvature to it. Should a straight and rigid series of reamers be employed to enlarge the canal, there is considerable likelihood that the reamer, in not being capable of following the bone's curvature, will not remove the desired uniform amount of bone tissue. In such a situation, excession tissue removal occurring in at least one plane will be experienced as the reamer is advanced. For this reason, medullary canals are almost always prepared with reamers having a flexible shaft.
These flexible medullary reamers consist of spiral or helically wound metal wire(s) or strip(s), which comprise the shaft of the reamer. A disadvantage of this design is that these reamers can be operated only in the forward or clockwise mode of rotation. If operated in the reverse or counter-clockwise mode, which occasionally is required to free a lodged reamer and to facilitate normal removal, the shaft unwinds, thus rendering the reamer permanently deformed, unusable, and unrepairable. This adds considerably to the cost of maintaining a serviceable set of medullary reamers. Further, a lodged cutting head may subsequently be extremely difficult, if not impossible to remove without further violation of the involved bone and surrounding tissues.
Another disadvantage of said design is the extreme difficulty in their proper and thorough cleaning after use. The spiral or helically wound metal shafts contain many voids of various sizes. Blood and tissue readily infiltrate such voids and become trapped within the confines of the shaft. When the reamer is in use, the voids are considerably distorted and enlarged as the reamer is advance towards and within the medullary canal, thus providing ready access for the particles of tissue. Prior to use, all medullary reamers are sterilized and hopefully, the blood and tissue particles not evacuated during the cleaning process and remaining within the interstices of the reamers, are at least rendered harmless. However, depending upon the amount and composition of the extraneous particles and their degree of isolation from the sterilizing process, said particles may not be rendered sterile. Even in a sterile condition, these foreign particles may still cause problems of infection should they become dislodged from the confines of the reamer and come into contact with the patient's internal tissues. Medical professionals recognize this problem but acquiesce to using these reamers for lack of an acceptable alternative.
A further disadvantage of this medullary reamer is that the torsional load it is subject to when in use results in poor power transfer and varying degrees of distortion of said shaft. If the power source providing the rotational energy to the reamer is great enough, said coils may tighten sufficiently to adversely affect the intended flexibility of the shaft. Another disadvantage associated with a spiral or helically wound reamer is the trauma it imposes to surrounding tissues. This results when the shaft of the reamer is not completely within the medullary canal as would occur during the initial reaming process. As the shaft rotates, that portion remaining outside of the medullary canal can become excessively flexed and distorted, thus enlarging the voids between the coils of the shaft. As the flexed shaft rotates, tissue lying outside of the canal and unintended for removal, becomes trapped within the voids and are torn from their underlying structures.